Chemical fume hood



Aug. 25, 1953 s w AL 2,649,727

CHEMICAL FUME HOOD Filed Jan. 4, 1951 2 Sheets-Sheet 2 /NVENTOR5 BOA/AL 0 L. 5/vovz/ Patented Aug. 25, 1953 CHEIWICAL FUME HOOD Donald L. Snow and Cyril S. Staub, Washington, D. 0.

Application January 4,1951, Serial No. 204,452 g 6 Claims. (01. 98-115) (Granted under Title 35, U. S. Code (1952),,

sec. 266) The invention described herein may be manufactured and used by or for the Government of the United States for governmental purposes without the payment to us of any royalty thereon in accordance with the provisions of the act of April 30, 1928 (Ch. 460, 45 Stat. L. 467).

This invention relates to a fume hood structure and method of operating such hoods. Still more particularly it relates to a hood structure and hood operation inair-conditioned spaces.

When ordinary hoods are operated airconditioned spaces, the hood exhaust 15 constantly removing large quantities of the cond tioned air from the room. This conditioned air is not recovered and places an unusually heavy load on the air-conditioning equipment so that in many instances the air conditioning is economically impractical.

Accordingly, it is an object of this invention to provide a new type of hood structure which can be economically used in air-conditioned laboratories, kitchens, and like spaces.

It is still another object to provide a method for operating hoods and the like, particularly in air-conditioned spaces. I

Other objects and advantages will be apparent or will appear hereinafter.

The invention will be better understood by reference to the drawings wherein:

Figure l is a vertical sectional representation of a hood and its operation.

Figure 2 is a horizontal section taken on lme AA of Figure 1.

Figure 3 is an isometric view of an embodiment of a hood in combination with a laboratory work bench and storage chamber, and

Figure 4 is a more detailed and enlarged view of a portion of Figure 1.

Referring now to the drawings, there is an enclosed hood working space In above the hood working surface or table II. Beneath the working surface there may be the usual laboratory storage space l2 for apparatus, chemicals and the like. The hood working space is enclosed on the sides by side panels [3 and the rear exhaust plenum 3| closes the back, and a top 14 completes the hood enclosure. Access to the working space 1s had from the open frontwhich can, if des1red, be closed off by transom or sash 15, as is cuss tomary in conventional laboratory hoods.

As shown in Figure l, a raw air (nonconditioned air). supply duct;20 carries air from a suitable supply source such as a blower (not shown) and terminates in aprimary air supply plenum or duct 2l.. Plenum 2i extends coinpl telra d he, 91 I??? 1. 1 PWQTQ??? sufiiciently above the working surface I I to permit access to the hood interior or working space Ill. The plenum opens into the hood through a resistance element 22 in the plenum. The resistance element is inserted to insure an even distribution of the air issuing from the plenum across the entire hood face; it can be formed by a perforated plate, screening, or the like. In the particular embodiment represented by Figure 1, the plenum outlet lies in a plane parallel to the hood working surface. This can readily be varied to suit individual applications; however, for most purposes the horizontal outlet will permit greater exclusion of air from the surrounding room space.

Still referring to Figure 1, there is shown a secondary air supply plenum or duct 23 extending across the edge of the working surface H which is adjacent to the open hood face. This plenum is preferably connected to the primary air plenum 2| through a plenum duct which is not shown in this particular figure but the equivalent of which can be seen in Figure 3 as side plenums 21 of the'plenum loop. However, a secondary air supply duct can be connected directly to this plenum if desired. In such cases the plenum should be of U shape and extend upwardly along both sides of the hood adjacent to the open face in order to provide for an air supply for the vertical slots as will be hereinafter described.

The secondary air plenum opens to the hood through a horizontal slot 24 and through vertical slots 26. The horizontal slot extends completely across the hood working surface adjacent to the open face, and is angularly displaced from the perpendicular so that it will direct any air issuing therefrom into the hood proper. As shown in Figure 4. the vertical slots 26 run up the side walls I3 of the hood, and as shown in the drawings can be considered a continuation of the horizontal slot 24. They need not be in an absolutely vertical plane but can be angularly disposed with relation to the working surface H, nor do they have to be a continuation of the horizontal slot. Any air issuing from plenum 23 through slots 26 is directed towards the rear of the hood by baflle or face plates 25. The vertical slots extend upwardly for a distance slightly beyond the point where the downwardly moving air: from the primary supply plenum curves into the hood proper due to the differential in pressure between the intake plenum and the exhaust baflle openings.

' As shownin Figures 1, 2, and 3 in the rear of the hood enclosure, there is an .exhaust'plenum 3| leading to an exhaust duct 30 which is coupled to an exhaust blower (not shown). This plenum has a panel 32 extending from the hood top nearly to the working surface II. The air inlet to the exhaust plenum is provided by means of the space or air gap 36 above the working surface I I In order to channel the fast-moving, even flowing air mass from the primary air plenum 2| to intercept that portion of the hood where maximum fumes are ordinarily produced (as shown in Figure 1 by the arrows indicating the desirable conditions of air movement) panel '32 is provided with perforations 33. A few such perforations are utilized in the top of the panel in order to exhaust the top of the hood in order to prevent an area of nonmoving atmosphere. The perforations utilized in the lower section of panel 32 are preferably spaced in rows with the vertical spacing of each row increasing as the height of the perforation above the work ing surface increases. The spacing, size, and distribution of these perforations will vary with design conditions. As shown in Figure l, the perforated panel 32 is inclined so as to accommodate a larger volume of air as it approaches the exhaust duct.

As shown in Figure 1, an exhaust duct 34 can also interconnect the storage space l2 and the exhaust plenum 31. A cutout or equivalent duct closure 35 can be installed in this line. This will permit the storage of fuming materials in the space below the working surface rather than within the hood enclosure as is quite often done in laboratories.

The operation of hoods of this type can be understood from the following example describing an actual hood installation; the invention, however, is not to be construed as limited thereto. The dimensions noted therein are approximate and are employed merely for simplicity of description.

Example In a hood having a face opening approximately 3' x (from the plenum outlet to the working surface), air ranging from 15 F. be low room temperatures and above was supplied at the rate of approximately 375 C. F. M. through supply duct 20. The air in the plenums was under a static H2O pressure of between 0.04" to 0.07". The primary plenum outlet Was about 4" wide and 5' long. The resistance element comprised a plate containing small circular perforations the total area of the perforations comprising approximately 44% of the area of a portion of the plate contained in a unit area. The horizontal slot outlet was directly below the primary plenum outlet and was about /8" wide by 5 long. The vertical slots were extensions of the horizontal slot and were about A;" wide and extended 18" high as measured from the horizontal slot. Air issued from the primary or upper plenum at a rate of about 330 C'. F. .M. and from the secondary or lower plenum at about 45 C. F. M. The exhaust was operated at a rate of about 500 C. F. M. Under these conditions, a downwardly and inwardly moving air "curtain was formed across the face of the hood as shown by the current arrows in Figure 1. The air from the horizontal and side slots converge therewith within the hood closure. Only about 125 C. F. M. of conditioned air was drawn through the hood face. This .has been found sufiicient to prevent fuming or smoking materials within the hood from escaping from the 4 hood into the surrounding room. Under conventional hood operation about 750 C. F. M. of conditioned air must be drawn across such a hood face for efficient hood operation. Puffiness or bucking resulting from converging air currents was held to a minimum when the aforementioned dimension and air rates were employed.

The horizontal slot was set at about a 45 angle. Fairing 16 was utilized to limit the forward edge of the working surface to insure that the equipment being used by the laboratory worker was within the air curtain. The fairing also serves the purposes of preventing any dead air area at this point and of preventing any spillage of reagents from leaking into the horizontal slot.

During hood operation the sash Hi can be within an inch or two of being closed or varied in position, if desired, without affecting the operation appreciably. When it is completely closed, the exhaust must be throttled down to be approximately equal to the supply if it is desired to maintain normal atmospheric pressure within the hood.

Under some laboratory conditions, it may be desirable to stop the air flow of nonconditioned air through the hood. For use under such conditions, the hood assembly may be provided with a short circuiting duct (not shown) provided with a suitable valve in order to shunt the raw air from the intake duct leading to plenums 2| and 23 directly to exhaust duct 30.

From the foregoing description, drawings, and example, it is apparent that a new type of hood structure and method of hood operation has been presented.

While the foregoing description has been limited to laboratory hood operations in air conditioned laboratories, the invention has many other applications. It can be used in kitchens, industrial operations, and like instances where objectionable fumes, smoke, odors, and the like result from the treatment or storage of materials. Installations of this type are effective regardless of whether or not the laboratory is air conditioned but the savings which result from its use in air-conditioned spaces make it especially adaptable for such purposes.

Since many widely differing embodiments of the invention will occur to persons skilled in the art, the invention is not limited to the specific details illustrated or described, and various changes can be made therein without departing from the spirit and scope thereof.

What is claimed is:

1. A method for operating a fume hood which comprises discharging a thick primary air curtain of downwardly moving air across an open hOO e; c ing a secondary air curtain of less thickness upwardly and inwardly into said hood, whereby the primary curtain is of substantially greater volume than the secondary curtain; and exhausting the air within the hood from .a portion of the hood opposite the air curtains at a rate slightly greater than the combined air supply rates of the primary and secondary curtains.

2. A method for operating a fume hood which comprises discharging a thick primar air curtain of downwardly moving air across an open hood face; discharging a secondary air curtain .of less thickness upwardly and inwardly into said hood, whereby the primary curtain is of substantially greater volume than the secondary curtain; and exhausting the air within the hood from a portion of the hood opposite the air curtains at a rate slightly greater than the combined air supply rates of the primary and secondary curtains and in such a manner that the major exhaustion occurs in the space immediately above the hood working surface.

3. The combination with a work table of a fume hood comprising a housing for the table havin an access opening extending across the front thereof above the table, a primary air duct extending along the top of said access opening for the full length thereof and having a similarlyextending, relatively-wide discharge opening for directing a stream of air downwardly across the face of said access opening, air-distributing means mounted in said discharge opening and cooperating therewith to effect an equalized discharge of said air stream in the form of a thick even curtain, a secondary air duct connected to said primary air duct and extending along the bottom of said access opening, said secondary air duct having a discharge opening extending along the bottom of said access opening for the full length thereof, said discharge Opening in the secondary air duct being of substantially less width than the discharge opening of said primary air duct, nozzle means formed above the opening of said secondary air duct for directing a curtain of air from said opening upwardly and toward the rear of said housing, means for moving air through said ducts, and means in the rear of said table for receiving and exhausting the air.

4. The combination with a work table of a fume hood comprising a housing for the table having an access opening extending acros the front there-of above the table, a primary air duct extending along the top of said access opening for the full length thereof and having a similarly-extending relatively-wide discharge opening for directing a stream of air downwardly across the face of said access opening, air-distributing means mounted in said discharge opening and cooperating therewith to effect an equalized discharge of said air stream in the form of a thick even curtain, a secondary air duct connected to said primary air duct and extending along the bottom and sides of said access opening, said secondary air duct having a discharge opening extending along the bottom of said access opening for the full length thereof, said discharge opening in the secondary air duct being of substantially less width than the discharge opening in said primary air duct, nozzle means formed above the opening of said secondary air duct for directing a curtain of air from said opening upwardly and toward the rear of said housing, said secondary air duct having discharge openings extending vertically upward along the sides of said access opening, bafile means mounted over said vertical discharge opening to direct an air stream therefrom toward the rear of said housing, means for moving air through said ducts, and means at the rear of the table for receiving and exhausting the air.

5. The combination with a work table of a fume hood comprising a housing for the table having an access opening therein extending across the front thereof above the table, a primary air duct extending along the top of said access opening for the full length thereof and having a similarly-extending relatively-wide discharge opening for directing a stream of air downwardly across the face of said access opening, a screen mounted in said discharge opening and cooperating therewith to effect an equalized discharge of said air stream in the form of a thick even curtain, a secondary air duct connected to said primary air duct and extending along the bottom of said access opening, said secondary air duct having a discharge opening extendin along the bottom of said access opening for the full length thereof, said discharge opening in the secondary air duct being of substantially less width than the discharge opening of said primary air duct, nozzle means formed above the opening of said secondary air duct for directing a curtain of air from said opening upwardly and toward the rear of said housing, means'for moving air through said ducts and means in the rear of said table for receiving and exhausting the air.

6. The combination with a work table of a fume hood comprising a housing for the table having an access opening therein extending across the front thereof above the table, a primary air duct extending along the top of said access opening for the full length thereof and having a similarly-extending relatively-wide discharge opening for directing a stream of air downwardly across the face of said access opening, air-distributing means mounted in said discharge opening and cooperating therewith to effect an equalized discharge of said air stream in the form of a thick even curtain, a secondary air duct connected to said primary air duct and extending along the bottom and sides of said access opening, said secondary air duct having a discharge opening extending along the bottom of said access opening for the full length thereof, said discharge opening in the secondary air duct being of substantially less Width than the discharge opening in said primary air duct, nozzle means formed above the opening of said secondary air duct for directing a curtain of air from said opening upwardly and toward the rear of said housing, an air exhaust duct opening into the top rear of said housing, baiiie means extending acros the back of said housing within the housing enclosure and spaced from said table to provide a primary air exhaust near the surface of said table and having perforations therein to provide secondary air exhausts in the upper portion of said housing, and means for moving air through said ducts.

DONALD L. SNOW. CYRIL S. STAUB.

References Cited in the file of this patent UNITED STATES PATENTS 

